1. Field of the Invention
This invention relates to a method for plasma-initiated polymerization. More particularly, this invention relates to a method for plasma-initiated polymerization which has a relatively short polymerization time, gives a desired product in a high yield, and abounds in practicality. To be more specific, this invention relates to a method for plasma-initiated polymerization which is capable of preparing an ultra-high molecular weight polymer preeminently excelling in such polymer properties as thermal properties and solution properties in a short polymerization time in a high yield.
2. Description of Prior Art
Polymers obtained by low-temperature plasma-initiated polymerization differ widely in nature from polymers obtained by heat-initiated polymerization. In various fields, such polymers by the plasma polymerization have come to attract growing attention in recent years. The plasma polymerization has been chiefly used in vapor-phase polymerization or applied for surface treatment. More recently, the so-called plasma-initiated polymerization which resides in utilizing the active seed produced in a vapor phase under generation of plasma for a condensatephase reaction has been established. The plasma-initiated polymerization comprises exposing a monomer vapor equilibrated with a liquid or solid monomer to a plasma for a short time thereby producing a polymerically active seed and then allowing the polymerically active seed produced in the plasma to come into contact with the surface of the liquid or solid monomer thereby starting polymerization growing in the liquid or solid monomer. Unlike the ordinary plasma polymerization such as the plasma vapor-phase polymerization wherein all the component steps including start of polymerization, growth, migration, recombination, and restart take place in the presence of plasma, the plasma-initiated polymerization in such that only the initiation of polymerization occurs in the vapor phase and the subsequent reactions for growth and stop of polymerization occur in the condensate phase.
After the plasma-initiated polymerization, post-polymerization can occur with living-radical monomer initiator. Therefore, the plasma-initiated polymerization is capable of producing a linear ultra-high molecular polymer without impairing the chemical structure of the starting monomer. This polymerization, therefore, permits ready analysis of its reaction mechanism and promises to permit development of varying functional high molecular compounds such as, for example, extremely high molecular polymers, block copolymers, immobilized enzymes, monocrystalline polymers, and inorganic polymers.
In the plasma-initiated polymerization of a vinyl monomer such as, for example, methyl methacrylate (MMA), when MMA is exposed to the plasma for 60 seconds and then subjected to post polymerization at 25.degree. C., the reaction of polymerization living-radically proceeds with elapse of time and reaches the level of 10,000,000 to 30,000,000 of molecular weight in ten days ([Y. Osada, A. T. Bell, and M. Shen: J. Polym. Sci., Polym. Lett. Ed. 16 (1978), 309], [D. Johnson, Y. Osada, A. T. Bell, and M. Shen: Macromolecules 14 (1981), 118], and [Y. Kashiwagi, Y. Einaga, and H. Fujita, Polym. J. 12 (1980), 271]. The polymethyl methacrylate (PMMA) obtained by the conventional thermal polymerization excels in transparency and nevertheless suffers from the disadvantage that thermal properties are poor, mechanical properties such as strength at rupture are low, solution properties are inferior, and elongation of polymer relative to solution concentration (thread-forming property) is small. In contrast, the polymethyl methacrylate (PMMA) obtained by the plasma-initiated polymerization described above has a molecular weight of the order of 10.sup.7, about 10 to 100 times as large as the molecular weight of the polymethyl methacrylate obtained by the conventional thermal polymerization. The plasma-initiated polymerized PMMA excels not only in mechanical properties at elevated temperatures but also in abrasion resistance and scratch hardness. Further, since it contains no additive except the initiator, it enjoys outstanding safety and has a bright prospect of finding utility in applications to medical high polymers.
As regards the microscopic structure, the plasma-initiated polymerization PMMA, on analysis by high resolution NMR, shows a triad distribution of iso, hetero, and syndio components. We have studied the plasma-initiated polymerized PMMA in comparison with the thermally polymerized PMMA and consequently found that it has a fairly high syndio component even with due allowance for the fraction ascribable to the factor of temperature and that it induces a polymerization reaction yielding a product of high crystal phase structure.
The method of plasma-initiated polymerization is capable of producing an excellent and characteristic polymer as described above. As noted from the literatures cited above, however, this method produces a polymer in a low yield and necessitates a very long time for polymerization (in the case of MMA as a monomer, for example, the yield is about 40 to 50% and the polymerization time is about 100 hours). Thus, the method of plasma-initiated polymerization proves to be quite disadvantageous from the viewpoint of practicality.
An object of this invention, therefore, is to provide an improved method of plasma-initiated polymerization.
A further object of this invention is to provide a highly practical method of plasma-initiated polymerization which has a relatively short polymerization time and produces polymer in a high yield.
Another object of this invention is to provide a method of plasma-initiated polymerization which is capable of shortening the polymerization time and increasing the yield of product of polymerization.
Yet another object of this invention is to provide a method of plasma-initiated polymerization which is capable of preparing an extremely high molecular weight polymer excellent in such polymer properties as thermal properties and solution properties with a short polymerization time in a high yield.